Hydrocarbons (oil, natural gas, etc.) are obtained from subterranean geologic formations by drilling a well that penetrates the hydrocarbon-bearing formation and seeks to release hydrocarbons trapped in the formation thus allowing them to reach the surface. In order for the hydrocarbon to be “produced”, the hydrocarbon must be able to travel from the formation to the wellbore (and ultimately to the surface) at a reasonably economic rate. Thus, there must be a sufficiently unimpeded flowpath from the formation to the wellbore. Hydraulic fracturing (“fracing”) is a primary tool for improving well productivity by placing or extending channels within the formation. This operation is essentially performed by hydraulically injecting a fracturing fluid into a wellbore penetrating a subterranean formation and forcing the fracturing fluid against the formation strata by extreme pressure thus inducing cracks or fractures in the formation.
Because the fractures would otherwise close upon release of the frac pressure due to the overburden weight, the cracks or fractures are held open by crush resistant “proppants” such as sand or ceramics included in the fracturing fluid. However, in delivering the proppants, which will fall out of solution if not suspended, fracturing fluids including proppants typically include viscosifying agents. Fracturing fluids are customized to the formations being fractured and, depending on the formation requirements, can be extremely viscous gel-like solutions or may be less viscous such as with foam or high flow-rate “slickwater” fracturing. Slickwater fracturing fluids are typically low viscosity, low proppant solutions pumped at very high rates.
Ideal fracturing fluids should be sufficiently viscous to create a fracture of adequate width, provide for maximal fluid travel distance to extend fracture length, be able to transport desired proppant amounts into the fracture, and require minimal gelling agent to allow for easier degradation or “breaking” at reasonable cost. The frac fluid will typically contain a number of other fluid additives in addition to the proppants in order to provide for formation clean up, foam stabilization, leakoff inhibition, and/or surface tension reduction. These additives include biocides, fluid-loss agents, enzyme breakers, acid breakers, oxidizing breakers, friction reducers, and surfactants such as emulsifiers and non-emulsifiers.
Natural gelling agents used in fracturing operations have included natural guar and locust bean gum, xanthan gum, starch and cellulose. Among the most commonly used polymers for fracturing are the high-molecular weight polysaccharides isolated from guar gums and derivatives thereof including hydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG) and carboxymethyl guar (CMG), hydrophobically modified guars, and guar-containing compounds. In addition to being highly viscous, the high-molecular weight guar based polysaccharides are essentially non-toxic. Guar, and derivatives thereof, are used both as viscosifying agents in frac fluids and as fluid loss control additives with low solid drilling muds.
After a fracturing fluid is formed and pumped into a subterranean formation and the proppant has been properly delivered into the fractures formed, it is generally desirable to convert the highly viscous fracturing fluid to a lower viscosity fluid that will not plug the formation. Residual unbroken polymers and filtercake can severely reduce permeability and conductivity in the fractured formation. The induced reduction in viscosity of the treating fluid is commonly referred to as “breaking.” Consequently, the materials used to break the viscosity of the fluid are referred to as breaking agents or “breakers.” Once broken, the polymer material flows easily out of the formation and desired material, such as oil or gas, is allowed to flow into the well bore. For purposes of breaking guar and guar based polymer gels, typically either oxidative, acid or enzyme breakers are used, each of which are directed to breaking connective linkages that form the highly viscous polymer chains.
Oxidative breakers generate free radicals that are able to attack the guar repeating unit at multiple sites and break the polymer chain. Typical oxidizers include persulfate (S2O2−) salts of ammonium, sodium and potassium, which decompose at elevated temperatures downhole to form free radicals that effect the breaking of the polymer chains. Because the breaking occurs rapidly at temperatures over 140° F., the use of the oxidizers must be carefully controlled to avoid breaking the gelling agents prior to proppant delivery. To avoid premature breaking, oxidizers are sometimes encapsulated. Acids such as hydrochloric acid are sometimes used as breakers. Both oxidizing and acid breakers can be corrosive, adversely reactive with both equipment and compounds used downhole, and can contaminate the produced oil and gas sufficiently to affect downstream catalytic processes.
Enzymatic breakers are particularly desirable as “green” non-toxic and biodegradable fracturing fluid additives. For breaking polymeric guar and derivatives thereof, endomannanases that catalyse the hydrolysis of 1,4-β-D-mannosidic linkages in mannans, galactomannans, and glucomannans may have applications in the aforementioned processes. Enzymes have been used for over 40 years as breakers but until relatively recently enzymatic breakers were not available that functioned optimally at the required pH and temperature conditions.
Taken together it is apparent that a frac fluid is a complex mixture that requires both adequate initial viscosity and the ability to be sufficiently broken after proppant deposition or placement. The present inventors appreciated that what are needed for a range of industrial applications are improved enzymes that are stable but less active under mixing conditions but are able to provide robust degradation of beta-1,4-linkages in viscous polymers under working conditions.